Fuses are widely used in semiconductor integrated circuits in both analog and digital circuits. For example, they can be used to store security codes, manufacturing data, device dependent data, redundant data, etc. A conventional integrated circuit might have 1000 or more fuses in its circuitry. Early fuses were metal based and were programmed by destroying the fuse with a laser beam. More recently, fuses have been developed that are electrically programmable using electromigration. See, for example, C. Kothandaraman et al., “Electrically Programmable Fuse (eFuse) Using Electromigration in Silicides,” IEEE Electron Device Letters, Vol. 23, No. 9, pp. 523-525 (September 2002). Typically, the state of the fuse—unprogrammed (unblown or intact) or programmed (or blown)—is read out by determining the voltage drop across the fuse using a voltage divider network or its equivalent. It has been difficult, however, to use electrically programmable fuses in modern integrated circuits because the fuses use a bias voltage that requires them to be relatively large in size and they have a narrow window of sensitivity that does not work well across process, voltage and temperature variations. Other circuits, such as those disclosed in the assignee's U.S. Pat. No. 7,304,527 B1, compare the resistance of the blown or unblown fuse with a reference resistance and generate an output signal indicating whether the fuse is blown or not. U.S. Pat. No. 7,304,527 is incorporated by reference herein in its entirety.
While the circuit of the '527 patent has several advantages over the prior art, it still conducts a significant amount of DC current (approximately 0.5 mA) during the sensing operation. As a result, in an integrated circuit with a large number of fuses, the use of circuits such as that of the '527 patent to sense the fuses imposes a significant current drain. Accommodating the sensing current also impacts the design of the power buses on the integrated circuit.